1. Field of the Invention
The present invention relates to a method of measuring spectroscopic absorbance such as Fourier transform infrared spectroscopy (FT-IR), and to an apparatus for measuring spectroscopic absorbance embodying the method.
2. Related Art
Conventionally, in the quality evaluation of a semiconductor producing process, FT-IR has been used to measure the concentration of substitutional carbon that is an impurity in a silicon single crystal. Specifically, a technique is known, in which differential spectroscopy is used to obtain an infrared absorption spectrum of substitutional carbon that is an impurity included in a measurement target (sample), by means of subtraction factor correction by subtracting an infrared absorption spectrum, which has been obtained from a substantially carbon-free silicon single crystal (reference), from an infrared absorption spectrum which has been obtained from a silicon single crystal that is the measurement target (sample), as disclosed in a Japanese Unexamined Patent Application, First Publication No. H06-194310 (Patent Document 1). The subtraction factor correction is used in order to remove the strong absorbance due to phonon absorbance of the silicon that is present overlapping with a minute spectrum of the substitutional carbon.
FIG. 7 is a diagram showing an example of a differential infrared absorption spectrum of substitutional carbon which has been corrected with a subtraction factor of the prior art. In this prior art, the range of the absorption peak of the substitutional carbon included in a difference spectrum is expediently set as the range of wave numbers from 595 to 615 cm−1 and a straight line passing both ends of this range is set as a base line 122, thereby obtaining a signal strength.
In addition, another prior art is known, which relates to a calibration curve correction method in near infrared spectroscopy. In the calibration curve correction method, a calibration curve created before the correction is applied to the corrected measurement spectrum. In this case, wave number shift correction is performed on the measurement spectrum to shift its wave number, and scale correction is performed for the wave number-shifted measurement spectrum, thereby applying the calibration curve to the corrected spectrum, as disclosed in a Japanese Unexamined Patent Application, First Publication No. 2007-18624 (Patent Document 2).